(1) Field of the Invention
The present invention generally relates to an exchange system having time switches and, more particularly, to an exchange system handling an in-band signal. An in-band signal is defined as a signal consisting of predetermined bits which are allocated in a PCM multiplexed signal transferred via a transmission path in order to transmit a line signal via the transmission line.
(2) Description of the Related Art
Recently, services presented to subscriber terminals have become more advanced. Thus, it is required that such advanced services be presented to subscriber terminals which are already coupled to the existing subscriber lines (or exchanges).
FIG. 1 is a block diagram of a communication system capable of presenting advanced services. The system shown in FIG.1 has exchange offices 3 and 5, both of which are connected to each other through a PCM transmission system 4. The exchange office 3 accommodates subscriber terminals 1 via subscriber lines 2, and the exchange office 5 accommodates subscriber terminals 9 via subscriber lines 10. Further, the exchange office 5 accommodates a pseudo subscriber circuit 8, which is connected to the exchange office 5 via a control line 6 and a subscriber line 7. Predetermined subscriber terminals among the subscriber terminals 1 connected to the exchange office 3 can receive a service presented by the exchange office 5 in the following way.
When the subscriber terminal 1 generates a call, the exchange office 3 creates a communication path A-A' between the subscriber line 2 and the transmission system 4, and informs the exchange office 5 of the generation of the call. The exchange office 5 has created a communication path B-B' between the transmission system 4 and the control line 6, and has activated the pseudo subscriber circuit 8 after the exchange office B restarts. It will be noted that the communication path B-B' is not created for each call but created on the basis of predetermined data (data about the specifically used line). The initial setting of the pseudo subscriber circuit 8 is carried out when the exchange office 5 restarts. The pseudo subscriber circuit 8 receives a speech signal and a line signal from the subscriber terminal 1 via the communication path A-A', the transmission system 4 and the communication path B-B'. The pseudo subscriber circuit 8 controls the subscriber line 7 in accordance with the line signal, so that the speech signal is transferred between the control line 6 and the subscriber line 7. That is, the exchange office 5 can execute the call processing procedure as if the subscriber terminal 1 is actually connected to the subscriber line 7. In this way, the subscriber terminal 1 can receive the service which is not provided by the office exchange 3.
Data (speech signal) is transferred on the transmission system 4 in a PCM 24 format, in which one multiframe consists of 12 consecutive frames, as shown in FIG. 2. Each of the 12 frames consists of 24 channels (time slots). Each channel consists of eight bits. The line signal transferred between the subscriber terminal 1 and the pseudo subscriber circuit 8 is transferred using the least significant bit (in-band signal) in each time slot of each of the sixth and twelfth frames 6F and 12F. The PCM-24 format shown in FIG. 2 is disclosed in, for example, Japanese Laid-Open Patent Application No. 64-18328.
As shown in FIG. 3, each of the exchange offices 3 and 5 has time switches #1 and #2, each of which is comprised of a memory. Information in the time slots (channels:CH) is alternately written into the memories #1 and #2 for every frame period. During operation, information is written into one of the memories #1 and #2, while information is read out from the other one of the memories #1 and #2. Thus, the order of frames obtained at the input sides of the memories #1 and #2 is the same as the order of frames obtained at the output sides thereof. As a result, in principle, the in-band signals placed in the specific frames can be exchanged in the same way as the speech signal.
However, the structure shown in FIG. 3 is not compact and simple because two memories are needed. In order to overcome this problem, an improved structure has been proposed in which a single time switch is used. However, there is a limitation regarding read timings of the single time switch. Thus, as will be described in detail later, there is a possibility that the in-band signal (least significant bits in the specific frames) may be located in a frame subsequent to the specific frame due to a delay in signal processing therein. The in-band signals which are not located in place cause various malfunctions of the system. For example, it becomes impossible to control the pseudo subscriber circuit 8 (FIG. 1).